southern ocean

One of the key uncertainties facing our current understanding of the Earth’s climate system is the role played by biology in the Southern Ocean. Observations show that nitrate, an essential macronutrient required by phytoplankton, is never fully consumed in Antarctic surface waters, likely due to iron limitation of phytoplankton in combination with unfavourable light conditions. Nitrate-fueled phytoplankton growth can be used as a measure of the flux of organic matter out of surface waters – this mechanism, which fixes atmospheric CO2 as biomass and exports it to the deep ocean, is termed the ‘biological pump’. The high nutrient-low chlorophyll state of the present-day Southern Ocean represents as a ‘leak’ in the global ocean’s biological pump since, by consuming nitrate more completely in surface waters, Antarctic phytoplankton have the potential to significantly lower atmospheric CO2. Understanding the controls on biological nitrate utilization in the modern Antarctic Ocean is central to our understanding of its outsized role in setting atmospheric CO2 today and in the past, and in absorbing CO2 in the future.

figure 1: Map of the July 2012 R/V SA Agulhas II cruise track along the Bonus GoodHope Line from Cape Town (CT) to the sea-ice edge (dashed white line), with the colour shading indicating July sea surface temperature (SST) climatology (in °C). The major fronts at the time of sampling are indicated (STF: Subtropical Front, SAF: Subantarctic Front, PF: Polar Front, SACCF: Southern Antarctic Circumpolar Current Front). The Antarctic Zone lies south of the PF, and is sub-divided by the Southern Antarctic Circumpolar Current Front (SACCF) into the northern Open Antarctic Zone (OAZ) and southern Polar Antarctic Zone (PAZ). Figure from Smart et al., 2015.

Nitrate-supported primary production (‘new’ production) in the Antarctic is also important for large-scale ocean fertility, fueling globally significant fisheries and supporting the large mammals and birds living in Antarctic waters. Given the expectation that diverse aspects of Southern Ocean physics and carbon chemistry are likely to change in the coming decades, a major motivation for working in the Southern Ocean is to develop expectations for Antarctic fertility and ecology in response to such changes.

I am involved in a collaborative effort with scientists from Princeton University, CSIR South Africa, and Stellenbosch University that seeks to characterize the seasonal cycle of productivity in Southern Ocean surface waters. Owing to a paucity of data from other times of year, our current view of the Antarctic N cycle is heavily biased towards the conditions characteristic of early- to mid summer. By leveraging South Africa’s unique position as a ‘gateway’ to the Southern Ocean, and using its state-of-the-art research vessel, the R/V SA Agulhas II, as a platform for combined research and education, our collaboration will extent this view of the Antarctic N cycle to include the winter, fall, and spring. Ultimately, we seek to address a persistent question in chemical oceanography, palaeo-climate, and carbon cycle science: what controls nitrate drawdown and export production in Antarctic surface waters?

Our first Antarctic cruise, which took place in July 2012, also happened to be the maiden voyage of South Africa’s new ice-breaker, the R/V SA Agulhas II (see figure 1 for cruise track). Graduate student, Sandi Smart, collected seawater samples along the entire GoodHope line and analyzed them for nitrate N and O isotopes, which record both physical and biological N cycle processes (figure 2). At present, we focus our interpretation on the Antarctic Zone (south of the Polar Front; PF).

Winter in the Antarctic Zone is typically considered biologically-dormant, with little but physical nutrient recharge via deep winter mixing affecting the nitrate pool in surface waters. However, we find that as in summer, nitrate assimilation appears to be the dominant control on the distribution of isotopes of nitrate. In contrast to the summer, the decoupling of the N and O isotopes of nitrate indicate that nitrification (the recycling of organic matter to nitrate) is also ongoing in wintertime Antarctic surface waters (figure 3), with implications for estimating organic carbon export based on nitrate-fueled phytoplankton growth.

The Antarctic surface ocean is likely conducive to nitrification because of its highly seasonal nature. Unlike the low-latitudes (where nitrification is largely confined to waters below the base of the euphotic zone), the Antarctic Zone is characterized by both a discrete productive summer period (which supplies the organic matter that will ultimately be regenerated to ammonium) and a low-light winter period with deeper mixed layers (which may free nitrifiers from both light inhibition and competition with phytoplankton for ammonium). In addition, the Antarctic Ocean has sea ice, which, with its high ammonium concentrations and more diffuse light, appears to be a favourable host environment for nitrification.

figure 3: Winter Antarctic Zone depth profiles in nitrate δ18O vs. δ15N space (OAZ in pink, PAZ in green). A solid grey 1:1 line passes through the average δ18O and δ15N of the deep nitrate source (assumed to be circumpolar deep water; CDW), illustrating the trajectory that the profiles would follow if nitrate assimilation were the only process occurring in the mixed layer. The blue arrows indicate the effect of in situ nitrification on Antarctic Zone mixed layer samples, projecting through the average measured δ18O and δ15N of mixed layer nitrate towards the δ18O and δ15N expected for newly nitrified nitrate (1.1‰ and -5‰, respectively). Figure from Smart et al., 2015.

Our nitrate N and O isotope data require that the organic matter available for nitrification in wintertime surface waters is low in δ15N (roughly -5‰). Such a low value can only derive from phytoplankton growth on recycled N (e.g., ammonium produced in surface waters during zooplankton and/or bacterial metabolism), which suggests that in the late summer/early fall, phytoplankton must switch from consuming nitrate mixed up from depth to consuming recycled N. This switch, which is the focus of our ongoing research, defines the point at which the upper ocean ecosystem no longer sequesters atmospheric CO2, with important implications for global climate and ocean fertility.